1. Field of the Invention
The present invention relates to a key element for the control of electromotors, especially for cranes and other hoisting equipment. The key element includes a key switch pestle having a permanent magnet attached thereto and being movable towards a sensing device and located within a housing for triggering the generation of a control signal by the sensing device.
2. Description of the Prior Art
An effect known as the Hall effect, after its discoverer, E. Hall, and an associated magnetoresistive effect form the physical basis for Hall sensors and magnetically-controllable resistor sensors. The combination of the Hall effect and the magnetoresistive effect show that moving charge carriers are diverted laterally under the influence of a vertical magnetic field, B, by the Lorentz force. Through the created transverse field, a state of equilibrium is established in the center of the semiconductor chip.
The Hall current U.sub.H is proportional to the magnetic field B and the control current i, and inversely proportional to both the layer thickness d and the charge concentration n of the strip being measured. R.sub.H is termed the Hall constant. When the current paths are observed, they are likewise found to be turned in the close vicinity of the current electrodes by the Hall angle. For small Hall angles (equal to or less than 0.45 degrees), a quadratic dependency of the resistance on the magnetic field B is found. R.sub.O is the resistance without a magnetic field.
Hall sensors are thus active quadrapoles with a current-voltage and Hall-voltage path, while magnetically controllable resistors represent passive magnetic-field-dependent resistances.
All magnetic field sensors that are not used for the direct measurement of magnetic fields utilize the magnetic field as a transmission medium between the physical variable to be measured (position, speed, current, output, etc.) and the sensor. The correct design of the magnetic circuit is thus a prerequisite for full utilization of the sensor characteristics. A distinction is made, depending on shearing, between an open (strongly sheared) and a closed magnetic circuit.
The simplest magnetic circuit consists of a permanent magnet and a sensor. Soft magnetic parts moving past the sensor lead to an induction change, which is detected by the sensor.
In constructing the sensor and selecting the air gap of the sensor/indicator, particular attention must be paid to the sharp drop in magnetic induction which occurs as the distance from the bottom surface of the magnet increases.
For the purpose of determining position, a closed magnetic circuit excited by a permanent magnet is frequently designed as a magnetic fork cabinet. Closed magnet circuits with electrical excitation form the basis for potential-free current and power measurement. The excitation which is generated by the current to be measured, provides an induction, which is measured by a Hall sensor in the air gap. A linear relationship is found between the measurement current and the induction.
In measurement and control engineering, position sensors serve to collect control parameters, such as path, angle and speed, in a non-contact and thus a wear-free manner. Hall sensors are especially suitable for this task.
For the present application of the Hall effect, high requirements are placed on the design of the magnetic circuit. These requirements include high permeability, good linear controllability, low remanence and a small air gap to name just a few.
Hall sensors are semiconductor components, the electrical behavior of which can be influenced by magnetic fields. The basis for this is the aforementioned Hall effect. This effect is seen in band-shaped conductors, through which current flows, which are exposed to a transversal magnetic field. Because of the magnetic field, the electrons flowing through the lamina in the longitudinal direction are diverted to the side. The Hall voltage can then be read off at the side contacts. This Hall voltage depends not only on the size of the current and the magnetic induction, but above all on the speed at which the electrons are flowing through the band-like conductor. In metals, the current flow is produced by many slow electrons, and the Hall voltage is so small as to be scarcely measurable. In contrast, in certain semiconductor materials, in which the current is carried by a few electrons of high speed, the Hall effect is very great.
Components based on this effect furnish an electrical signal when acted on by a magnetic field. If such components are connected to a permanent magnet, then their Hall voltage will change upon the approach of a soft magnetic body which influences the existing magnetic field. Compared to inductive pick-ups, the components based on the Hall effect have an advantage in that their signal voltage is independent of the speed at which the body to be registered, the magnetic body, approaches the sensor. In keeping with the different requirements for the sensor, different design forms may be used.
Key elements for the control of electromotors with multiple speeds should provide a greater or lesser speed depending on the actuating path. This is achieved, for example, in that the key switch pestle sequentially closes multiple contacts on its path, with which contacts assigned speed steps are set. For infinitely variable speed control, the key switch pestle can move a slider on a resistance path, for example. The varying electrical resistance picked up along the pestle path then sets a speed in a path-dependent manner. Both solutions, use of contacts or a slider, are accompanied by wear. This is because the key path is converted into electrical control signals by means of contacting touches. It would be advantageous to have contact-free position measurement for speed control through a key element, because this would not entail wear.
Furthermore, with respect to size, a key element for the control of electromotors should have external dimensions which allow it to fit comfortably into other devices, for example, hand controls and the like, without increasing the size of the device.